86 MOVEMENTS OF CURVATURE 



chemical stimulation causes them to press closely together, and hence also 

 against the body of a captured insect l . 



Since proteids, ammonium salts, phosphates and other substances act as stimuli, 

 the products of the digestion of a captured insect or of a piece of meat or egg- 

 albumin always induce a chemical excitation. Darwin found that ammonium 

 phosphate was more active than any other substance, for a drop of water containing 

 0-000423 of a milligram of this substance caused a curvature when placed upon 

 the head of the tentacle. The same result was produced by 0-0025 of a milligram 

 of ammonium nitrate and 0-0675 of a milligram of ammonium carbonate. Darwin 

 also found that phosphates, and to a less degree camphor, a few ethereal oils, and 

 in fact most varied substances acted as stimuli, but not certain alkaloids, so that 

 all substances containing nitrogen are not chemical excitants. Darwin observed 

 that the irritability of the tentacles of Drosera was suppressed by the application 

 of small amounts of potassium salts, and this has been confirmed by Correns 2 . The 

 latter author also finds that distilled water produces a feeble curvature, so that 

 it remains an open question to what extent the response or lack of response to 

 particular substances is due to external circumstances or to the presence of traces 

 of potassium salts. It is possible that the stimulating action of distilled water is 

 due to its dissolving away or diluting substances present in the glandular excretion, 

 which by causing a difference of concentration in regard to the cell-sap might induce 

 an excitation. In addition it is not sufficiently certain whether the inactivity of 

 certain substances is or is not due to their non-absorption. Since, however, in 

 general absorption is a preliminary to excitation, one may follow Munk 3 in speaking 

 of absorption stimuli and digestive movements, although this must not be taken to 

 indicate that only nutritive substances act as stimuli. 



The association of a mechanical with a chemical irritability is of 

 biological importance to carnivorous plants, although in other cases the 

 one form of irritability may be developed but not the other 4 . Tendrils 

 and the stamens of Cynareae are hardly or not at all responsive to chemical 

 stimuli, but are readily excitable by mechanical ones. Chemical stimuli 

 appear to have a more intense and prolonged action than mechanical ones 

 in the case of the carnivorous plants, and in fact the mechanical excitability 

 is so feeble in Drosera binata that it was overlooked by Morren 5 . It 



1 Darwin, 1876, 1. c., p. 307 ; Batalin, Flora, 1877, p. 134. 



2 Correns, Bot. Ztg., 1896, p. 25. 



3 Munk, Die elektr.- u. Bewegungsersch. an Dionaea, Reichert und du Bois-Reymond's Archiv, 

 1876, p. 98. 



4 Darwin (1875, 1- C was the first to distinguish between mechanical and chemical excitation. 

 The prolonged closure over insects was observed earlier, but was either unexplained or ascribed to the 

 continuance of the mechanical excitation. This explanation was, in fact, given by Oudemans (Bot. 

 Ztg., 1860, p. 163) in the case of the leaf of Dionaea. 



* Morren, Note sur le Drosera binata, 1875, p. 10 (reprint from Bull, de 1'Acad. royale de 

 Belgiqne, 2 e ser., T. XL). The mechanical excitability was detected by Darwin, 1. c., p. 256, and by 

 Goebel, Pflanzenbiol. Schilderungen, 1893, Bd. II, p. 199. Darwin (1. c., p. 270) also found that the 

 leaf of Drosera was still excitable by proteids when almost inexcitable by mechanical stimuli. 



